Unintended consequences: a) On Tuesday, Ireland, concerned about a run on its banks, guaranteed all their deposits
b) The next day, the Irish banks start calling UK bank customers and suggest they transfer money to their banks
c) On Friday, Greece guaranteed all of its banks deposits
You now have two Euro-denominated countries with unlimited deposit insurance and others without unlimited deposit insurance. Unless someone restricts the program, either the rest of Europe will have to do the same or you will see deposits flow to those countries, weakening the countries they are leaving from but ultimately destabilizing even the recipient countries.
These events either start to push the Euro apart or, more likely, force Europe to integrate more closely from an economic and regulatory perspective.